The cellular communications industry has experienced very rapid growth in both the areas serviced and the number of subscribers. It is not uncommon in many metropolitan areas for mobile switching centers (MSCs) to have service requests numbering in excess of 100,000 call attempts per hour. In these metropolitan areas, each MSC may serve a network of 100 or more cell sites, or base stations, each of which defines a particular cell. Each MSC also keeps track of all mobile units, such as cellular telephones, currently operating in its service area via a database commonly referred to as a visitor location register (VLR). This register stores information regarding the services available to each subscriber, his home MSC and the current status (e.g., active or inactive) of the mobile unit.
The MSC, however, does not usually have accurate information regarding the precise geographical location of each of the mobile units within the service area. Consequently, when a call is initiated to a mobile unit believed to be within the service area, a page must be broadcast over a forward control channel to all cells in the service area. When the mobile unit responds to the page, the particular cell containing the mobile unit is then identified from a reverse control channel used for the mobile response and a cellular link is then set up. In the event there is no response to the page, the system assumes that the mobile unit is currently inactive and the call is treated accordingly.
In order to broadcast a page over the forward control channels of all the base stations in a typical metropolitan system including an MSC serving 100 base stations, the page must be replicated 100 times and a copy sent to each of the 100 base station. As only one base station in the system can respond to the page, the vast majority of the pages are unnecessary, as they will result in no reply whatsoever. These unnecessary pages are produced at the expense of the MSC being available for performing other tasks. Hence, the overall system performance is degraded by the overhead required for the generation of unnecessary pages.
Various approaches have been suggested for overcoming this problem. In one such approach, the cellular network is partitioned into several location areas, each location area including a group of the cells covered by the MSC. Every time a mobile unit leaves one location area and enters another location area, it registers a location update that indicates the mobile unit is a visitor from the location area in which the mobile unit is registered. As a result, should a call be initiated to that mobile unit, the page needs to be sent only to the cells in the location area where the mobile unit is "visiting", thereby reducing system overhead associated with paging that particular mobile unit.
Another approach, discussed in pending patent application Ser. No. 08/743,689 filed on Nov. 6, 1996 and entitled "Method for Reducing Paging Load in a Cellular Communication System", the entire contents of which is herein incorporated by reference, is known as zone paging. In this approach, a two dimensional array referred to as a Location Accuracy Matrix (LAM) is developed. Each entry (i, j) to in the LAM represents the number of page responses received from mobile units in cell j when the last known location of each of the mobile units was in cell i. The LAM data is then used to develop a probability matrix (p(i, j)) that indicates the probability that a mobile unit is in cell j given the fact that its last known location was in cell i. The zone used for paging a mobile unit the last known location of which was in cell i is all the cells j for which the probability p(i, j) &gt;0.001. In the event that this selective zone page is unsuccessful, then all cells in the service area are paged in an attempt to locate the mobile unit. If the all zone page is unsuccessful, the mobile unit is presumed to be turned off.
Although the above-described approaches do reduce the number of pages required to find the cell in which a mobile unit is located, further reductions in the number of cells paged would directly benefit the operation of the MSC. Therefore, what is needed is a system that, with a relatively high level of certainty, only requires a single cell to be paged.
In addition to dealing with paging overhead, the MSC, in conjunction with its base stations, must accommodate the unique radio frequency (RF) characteristics of individual cells in its service area. For example, as a mobile unit moves about within a cell, certain areas of the cell may have obstructions or changes in environment that cause the mobile unit to lose contact with the base station. Such obstructions may be caused by a new building or new billboards that adversely affect the cellular link between the mobile unit and the base station. In addition, such obstructions may interfere with the handoff process when the mobile unit moves from one cell to another. Typically, the location at which the handoff occurs, that is, the "handoff threshold", is the midpoint between the two base stations involved. In actuality, however, the optimal handoff threshold is subject to the unique RF characteristics of each cell and can be affected by changes in the environment.
Therefore, what is needed is a system that automatically reacts to environmental changes in a dynamic service area to consistently provide the best service possible and to provide notification when changes in the environment require extra attention.
In addition, recent governmental regulations require service providers to be able to locate a mobile unit to within 125 meters.